In recent years, as radio communication increases in speed, high-efficiency transmission through digitalization has been employed. Under such a situation, a power amplifier is demanded to have high linearity in order to suppress degradation in signal quality. At the same time, from the viewpoint of miniaturization of the apparatus, reduction of operational cost, and the like, the power amplifier is highly demanded to operate with high power conversion efficiency. In general, there is a tradeoff relation between the linearity and the power conversion efficiency in the power amplifier.
In this regard, in order to achieve both the linearity and the power conversion efficiency, there is known a technique in which the power amplifier is operated in a non-linear area having high power conversion efficiency while linearity is maintained using distortion compensation for removing a nonlinear distortion generated at that time. A pre-distortion type technique (hereinafter, referred to as a “PD” type) as a sort of the distortion compensation improves linearity in the output of the power amplifier by configuring to have an anti-nonlinear distortion characteristic of the power amplifier in the transmission signal in advance. As an example of the PD type technique of the related art, there is known a power-series distortion compensation technique in which the anti-nonlinear distortion characteristic of the power amplifier is approximated using a power series.
Here, there is known a phenomenon called a memory effect which is generated in the power amplifier that operates with high power exchange efficiency. The memory effect is a phenomenon in which an output in response to an input of the power amplifier at certain timing is influenced by the previous input and includes an electric memory effect caused by an electric transient response of the power amplifier and a thermal memory effect caused by a thermal characteristic of the power amplifier. Due to such a memory effect, it is difficult to obtain a desired distortion suppression effect when the PD type distortion compensation is performed using a power series, in which the distortion compensation coefficient is determined only based on the current signal amplitude value or power value. In this regard, there has been proposed a PD type distortion compensation using the power series, in which the previous input signal is taken into consideration in addition to the current input signal in order to obtain a desired distortion suppression effect in the power amplifier that operates with high power efficiency. Furthermore, there has been proposed a PD type distortion compensation using a look-up table (LUT) for storing distortion compensation coefficients of the power series in order to reduce the number of computations for the power series.
Patent Literature 1: Japanese Patent No. 4086133
Patent Literature 2: Japanese National Publication of International Patent Application No. 2006-505160
Non-Patent Literature 1: A Flexible Volterra-Based Adaptive Digital Pre-Distortion Solution for Wideband RF Power Amplifier Linearization, Hardik Gandhi, Tex. Instruments, Palo Alto, Calif. 94306, USA
However, in the related art described above, it is difficult to appropriately perform distortion compensation while suppressing an increase in a circuit scale. For example, in the related art that uses the power series, a delay amount as a parameter of the power series increases as the distortion suppression effect increases in principle. For this reason, the number of delay circuits affecting the delay amount increases. This results in an increase in a circuit scale. This is similarly applied to the related art that uses the LUT. Furthermore, as the delay amount of the power series increases, the number of distortion compensation coefficients increases. Therefore, it is difficult to cause the distortion compensation coefficient to converge on an optimal value because the number of distortion compensation coefficients increases as the delay amount of the power series increases. This may degrade the distortion suppression performance.
In addition, in the related art using the LUT, a distortion compensation coefficient corresponding to a combination of the previous input signal and the current input signal input to the power amplifier is obtained from a single LUT, and the pre-distortion processing is performed using a single distortion compensation coefficient. Therefore, the distortion suppression performance may be insufficient.